1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that transports a stencil sheet corresponding to a recording medium and supplied between a head and a platen roller, by rotation of the platen roller, as the apparatus forms a perforated image on the stencil sheet.
2. Description of the Related Art
In known stencil printing machines in the past, a stencil sheet is melted in accordance with image information of an original to be printed, causing holes therein, the thus perforated stencil sheet being used to print the image information onto a printing paper.
In the above type of stencil printing machine, the formation of perforations in the stencil sheet by melting is done by a stencil making unit, which functions as an image forming apparatus for the perforation image.
The type of image forming apparatus studied by the inventor is described below.
In FIG. 1, a platen roller 1 is fixed by support shafts 2 that protrude on both sides thereof. The protruding ends of the support shafts 2 are rotatably supported by the main apparatus (not shown in the drawing). The support shafts 2 are linked to a platen rotational drive motor (not shown in the drawing), the rotational drive of the platen rotational drive motor causing the platen roller 1 to rotate.
A thermal head 3 (thermal printing head) melts the stencil sheet 4 in accordance with the image information to form perforations therein, and is fixed to a head support member 5.
The head support member 5 is swingably supported to the main apparatus by a shaft a about which it swings. The swinging of the head support member 5 freely moves the thermal head 3 with respect to the platen roller 1 in the contact and retraction directions indicated by the arrow A.
One end of a first spring 6 is engaged with the head support member 5. The spring force of the first spring 6 impels the thermal head 3 so that it is pressed against the platen roller 1.
A pressure adjustment cam 8 is fixed to the output shaft of a head pressure adjustment motor 7.
The pressure adjustment cam 8, in response to the force from the pressure adjustment motor 7, rotates freely in the direction shown by the arrow B, and the rotational position thereof establishes engagement and disengagement with respect to the head support member 5.
That is, when the pressure adjustment cam 8 rotates so that its longitudinal direction makes contact with the bottom surface 5a of the head support member 5 in the direction Z that is perpendicular to the Y direction, which is the width direction of the head support member 5 as shown in FIG. 1, the pressure adjustment cam 8 presses up against the bottom surface 5a of the head support member 5, and the head support member 5 resists the spring force of the first spring 6, rotating in the clockwise direction as shown in FIG. 1. Therefore, the thermal head 4 fixed to the head support member 5 moves to a position that is retracted from the platen roller 1.
When the pressure adjustment cam 8 rotates so that its longitudinal direction moves from the Z direction to the Y direction, the pressure of the pressure adjustment cam 8 up against the head support member 5 is released, so that, by means of the spring force of the first spring 6, the head support member 5 rotates in the counterclockwise direction shown in FIG. 1. Therefore, the thermal head 3 moves to a position that comes into pressure contact with the platen roller 1.
In the above-noted configuration, when the thermal head 3 is at a position that is retracted from the platen roller 1, the stencil sheet 4 is fed between the thermal head 3 and the platen roller 1.
Next, when the stencil making mode, this being the image forming mode, is selected, the head pressure adjustment motor 7 provides drive, so that the thermal head 3 moves from the position in which it is retracted from the platen roller 1 to the position at which it is pressed up against the platen roller 1, the stencil sheet 4 coming into contact between the thermal head 3 and the platen roller 1.
Next, the platen roller 1, in response to the speed of the image forming by the thermal head 3, is rotated by the rotational force of the platen rotational drive motor, as the thermal head 3 melts the stencil sheet 4 to form perforations therein in accordance with the image information of the original.
That is, the stencil sheet 4 is transported by the rotation of the platen roller 1, as the thermal head 3 successively forms perforations therein, responsive to the image information of the original, and at the point at which the entire image information of the original has been formed as perforations, the perforation image formation on the stencil sheet 4 is completed.
However, the inventor made a further study with regard to an image forming apparatus configured as described above, from which in the image forming mode, as shown in FIG. 2, because the thermal head 3 is in a position at which it presses up against the platen roller 1, the thermal head 3 indeed pressed thereagainst. The platen roller 1 receives the pressure applied at this time by the support shafts 2.
Because the support shafts 2 are supported at their two ends by the main apparatus, a bending moment occurs in the platen roller 1, so that the platen roller 1 sags in the negative Z direction, the maximum sag being at the center thereof, at which the deflection xcex41 occurs.
The thermal head 3 is influenced by this deformation of the platen roller 1, and deflects in the negative Z direction at center by an amount of xcex42.
In this case, because the maximum sag deflection xcex41 of the platen roller 1 is greater than the maximum sag deflection xcex4 of the thermal head 3, the spacing between the thermal head 3 and the platen roller 1 becomes uneven.
This has the effect of making the pressure on the stencil sheet 4 that is transported and pressed between the thermal head 3 and the platen roller 1 reduced at the center part thereof, at which part it is difficult to obtain a good reproduction of the image information of the original as a perforation image.
The present invention was made after the above-described study, and has as an object to provide an image forming apparatus which can maintain the pressure on the recording medium transported between the head and the platen roller as uniform, and is capable of obtaining good image generation characteristics at the recording medium.
An image forming apparatus according to the present invention is provided with a platen roller, a head capable of being pressed against the platen roller and forming an image onto a recording medium, a first impelling member impelling so as to press the head against the platen roller, an impelling roller capable of being pressed against the platen roller, and a second impelling member impelling so as to press the impelling roller against the platen roller. Here, while the head is pressed against the platen roller by use of the first impelling member and the recording medium supplied between the head and the platen roller is pressed, and while the impelling roller is pressed against the platen roller by use of the second impelling member, the recording medium is transported as the image is formed on the recording medium.